1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for generating an acquisition scheme for vibroseis marine sources.
2. Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, seismic sources are essentially impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources are airguns. The airguns produce a high amount of acoustics energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. The acoustic waves from the airguns propagate in all directions. A typical frequency range of the acoustic waves emitted by the impulsive sources is between 6 and 300 Hz. However, the frequency content of the impulsive sources is not fully controllable and different sources are selected depending on the needs of a particular survey. In addition, the use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources that may be used are vibratory sources. Vibratory sources, including hydraulically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in marine operations. However, there is no large scale use of such sources as they have limited power and are not reliable due to the number of moving parts required to generate the seismic waves. A positive aspect of the vibratory sources is that they can generate signals over various frequency bands, commonly referred to as “frequency sweeps”. The frequency band of such sources may be better controlled compared to impulsive sources. However, the known vibratory sources do not have a high vertical resolution as the typical frequency range of a marine seismic source represents approximately four octaves. A few examples of such sources are now discussed.
The vibratory sources need to be spatially arranged, when towed, so that they reasonably cover the subsurface desired to be investigated and also provide a high energy output so that the receivers are able to record the reflected seismic waves. Various arrangements are known in the art for impulsive sources that may also be used for the vibratory sources. For example, FIG. 1 shows a system 10 in which a source array 20 is towed underwater with plural streamers 30 (four in this case). The figure illustrates a cross-sectional view of this system, i.e., in a plane perpendicular to the streamers. The seismic waves 22a-d emitted by the source are reflected from a surface 40 and recorded by receivers of the streamers 30. A distance “a” between two successive reflections is called a bin size. Because this bin size is measured along a cross-line, “a” represents the cross-line bin size. The cross-line is defined as a line substantially perpendicular to the streamers, different from an axis Z that describes the depth of the streamers underwater. An inline is a line that extends substantially along the streamers and is perpendicular on the cross-line. For example, the Cartesian system shown in FIG. 1 has the X axis parallel to the inline, the Y axis parallel to the cross-line and the Z axis describes the depth of the streamers.
With this arrangement, the cross-line bin size is half the cross-line distance 42 between two consecutive streamers. It is noted that the streamers are typically placed 100 m from each other. The inline bin size may be much smaller as it depends mainly on the separation between the receivers in the streamer itself, which may be around 12 to 15 m. Thus, it is desired to decrease the cross-line bin size. With a cross-line bin size in the order of 50 m, aliasing effects may be produced, especially for the highest frequencies as the maximum bin size is inversely proportional to the frequency.
A common technique for reducing the cross-line bin size is the flip-flop acquisition scheme. In this mode, the vessel tows two sources 20 and 20′ as shown in FIG. 2. This arrangement 50 is configured to shoot one source 20, listen for a predetermined time for the reflections of the first emitted wave, and then to shoot the other source 20′ and listen for the reflections of the second emitted wave. Then, the process is repeated. This scheme doubles the coverage and reduces the cross-line bin size to a distance “b”, which is smaller than “a”.
However, due to the particulars of the vibro-acoustic sources, there are additional acquisition schemes, not applicable to impulsive sources, that can be used to increase the performances of the acquisition as discussed next.